Transfusion of red blood cells (RBCs) is routinely used for many clinical and surgical applications. On average, 39,000 units of blood are needed every day, and data from 2004 indicate that 29 million units of blood were transfused in one year (American Association of Blood Banks website). This procedure has single-handedly saved many lives over the past 60 years. The demand for such transfusions continues to increase with advances in medical treatments and an aging population.
In addition to the traditional clinical settings that have benefited from the availability of red blood cell transfusion, such as surgery and treatment of trauma patients, there are a number of unique instances in which red blood cell transfusion would change the standard of care. For example, there are a number of rare phenotypes of RBCs in patients of Afro-Caribbean descent (Douay et al., Transfusion Medicine Reviews 21, 91-100, 2007). They are considered rare phenotypes, due to the lack of antigens such as H or ABO blood groups. Such patients can develop a neutralizing antibody response to ABO blood group antigens, rendering them ineligible for RBC transfusions. In fact, such patients must receive transfusions from an identical source to avoid a neutralizing antibody response, posing tremendous challenges in cases where repeated red blood cell transfusions are required (e.g., sickle cell patients, etc.).
Additionally, patients who suffer from a variety of antibody-based autoimmune diseases and experience autoimmune hemolytic anemia may also benefit from red blood cell transfusions. However, this presents a challenge in finding a donor, or limited set of donors whose RBCs are compatible with the patients' autoantibodies. In essence, these patients experience the same challenges as those with rare blood phenotypes.
Moreover, patients who suffer from hemoglobinopathies and thalassemias have congenic mutations that result in a shorter life span for their RBCs. While the idea of improving the life and health of these patients with blood transfusions is an old one, the frequency of transfusions required presents a major problem. The average lifespan of RBCs from a healthy donor is 28 days. The number of transfusions required for these patients is large, frequent, and poses a significantly increased risk of iatrogenic infection. The ability to generate RBCs in vitro and to provide transfusions of synchronized RBCs with a mean lifespan of 120 days would greatly reduce the number of transfusions required for these patients and truly improve their quality of life.
The issue of lifespan of RBCs collected from donors is also important in the context of traditional clinical use of RBCs for trauma and surgical procedures. The storage of RBC concentrates for up to one month may result in an RBC population that requires at least 24 hours to recover its ability to transport oxygen. In addition, a number of necrotic RBCs in those concentrates could trigger an inflammatory response in the recipient, along with the complications that arise from such an inflammatory response. The ability to generate a constant supply of RBCs in vitro would allow health professionals to anticipate and to meet the demands for fresh RBCs, and would also eliminate the need for long-term storage of RBC concentrates
Another problem with red blood cell transfusions is the increasing difficulty in providing red blood cell transfusions. The reasons for this increasing difficulty include a steady drop in the supply of donated blood that is eligible for transfusion due to the increased number of infectious agents that have been shown to be transmitted through blood transfusions, the failure of hemoglobin and oxygen transporters (perfluorocarbons) to show efficacy as RBC alternatives in the clinical setting, and recent complications associated with erythropoietin (EPO) usage. Ready access to a continuous supply of RBC progenitors that could generate a defined RBC product for transfusion would alter the practice in the clinic and render blood transfusion a safer and more extensively used procedure. However, such an approach must be able to provide a supply of RBCs that is safe, effective, and universal.
While some initial attempts have been made to derive RBCs in vitro from primary hematopoietic stem cells (derived from bone marrow, cord blood, or peripheral blood) or embryonic stem cells, to date they have been unsuccessful for a variety of reasons including one or more of expense, duration of protocol, multiple steps, use of feeder cells or serum, labor intensiveness, low yield, or failure to fully differentiate to mature, anucleated red blood cells. Attempts to generate RBCs in vitro include methods starting from primary hemotopoietic stem cells (Neildez-Nguyen et al., Nat Biotech 20, 467-72, 2002), and embryonic stem cells (Lu et al., Blood. 2008 Dec. 1; 112(12):4475-84; Lu et al., Regen Med 3, 693-704, 2008. These approaches also don't generally allow for a defined and continuous source of RBC progenitors.
Citation of the above documents and studies is not intended as an admission that any of the foregoing is pertinent prior art. All statements as to the contents of these documents are based on the information available to the applicants and do not constitute any admission as to the correctness of the contents of these documents.